1. Field of the Invention
The present invention relates to an aqueous hydraulic cement slurry composition especially suitable for use in cementing a well.
More particularly, the present invention concerns polymaleic anhydride additives for hydraulic cements used in cementing gas and oil wells which improve the flow properties of the cement slurry, especially facilitating high flow rates and turbulent flow of the cement slurry when it is being pumped to its desired location during the cementing operation.
As used herein, the terms "improve the flow properties" or "flow-property-improving" are intended to describe the manner in which the polymaleic anhydride additive compositions of the present invention facilitate or render more efficient the pumping of cement slurries, particularly the pumping of well cement slurries during primary cementing operations. This action is apparently due primarily to a reduction in the frictional drag experienced by the cement slurry while it is being pumped or moved through a confining passageway, but it is not intended to limit the described action to such a friction reducing effect, since other effects may also play a role.
The term "turbulence-inducing" is intended to describe the effect of the polymaleic anhydride additive compositions of the present invention in promoting the departure of a moving cement slurry from laminar flow to turbulent flow. The slurry in such a state of flow no longer moves in laminae aligned along the confining conduit, but experiences a disorganized flow characterized by eddies and disturbances. The flow-property-improving and turbulence-inducing additives of the present invention reduce the apparent viscosity of a cement slurry, which in turn reduces the flow rate required to cause turbulent flow. Thus, the promotion of turbulent flow may take place simply as a reduction in the amount of pumping pressure or volume necessary to induce a state of turbulent flow for a particular cement slurry under specific pumping conditions, below that which would be required if the turbulence-inducing additive were omitted from the cement slurry composition.
Techniques for drilling and completing wells, particularly gas and oil wells, are well-established. Of chief concern here are those wells which are drilled from the surface of the earth to some subterranean formation containing a fluid mineral which it is desired to recover. After the fluid-containing geologic formation is located by investigation, a bore-hole is drilled through the overlying layers of the earth's crust to the fluid-containing geologic formation in order to permit recovery of the fluid mineral contained therein. A casing is then secured in position within the bore-hole to prevent entry into the well of a fluid from a formation other than the formation which is being tapped. This well casing is usually cemented in place by pumping a cement slurry downwardly through the well bore-hole, which is usually accomplished by means of conducting tubing within the well casing. The cement slurry flows out of the open lower end of the casing at the well bottom and then upwardly around the casing in the annular space between the outer wall of the casing and the wall of the well bore-hole. The drilling process which produces the bore-hole will usually leave behind on the wall of the bore-hole produced, a drilling fluid filter cake of mud-like material. This material is a barrier to the formation of proper bonding by any cement composition employed to produce an impermeable bond between the casing and the well wall. As a result, cementing operations have often proven inadequate, permitting fluids from other formations to migrate into the producing formation, and vice versa. Prior art solutions to this problem have included washing away the filter cake from the well wall prior to the cementing operation. However, the washing liquids themselves have introduced new problems, including reduced permeability of the producing formation.
Nevertheless, an effective cementing operation requires that the drilling fluid filter cake be removed from the well bore wall and replaced by the cement slurry in order to permit the formation of a solid layer of hardened and cured cement between the casing and the geologic formations through which the well bore-hole passes. It has been recognized in the art that removal of the drilling fluid filter cake may be accomplished by a sufficiently high flow rate for the cement slurry during its injection into the well bore-hole. Such a high flow rate will usually occur as turbulent flow.
The flow properties of cement slurries are also important during primary cementing operations in other respects. The pressure drop in the annulus being cemented which results from friction will increase both the hydraulic horsepower required to move the cement slurry into place in a given time period, as well as the hydrostatic pressure exerted on the producing formation. Moreover, some investigators have felt that obtaining actual turbulent flow of the cement slurry is not necessary to remove the drilling fluid filter cake, and that high flow rates are sufficient where the drilling fluid has low gel strength and there is good centralization (concentric placement of the well casing within the well bore-hole). Consequently, the present invention is directed to well cementing compositions having improved flow properties, including, but not limited to, adaptability to turbulent, especially high turbulent flow.
Prior to the discovery of flow-property-improving, turbulence-inducing additives for well cement compositions, achieving high flow rate and turbulent flow for conventional cement slurries presented a number of problems not normally encountered with less viscous fluids. Inducing turbulence by control of flow rate alone has required a certain minimum velocity, which in turn is dependent upon maintaining a certain minimum pressure. Particularly, where the turbulence induced is sufficient to assure removal of the drilling fluid filter cake, additional pumping capacity and very high pressure levels are required. Producing high flow rates has, of course, required the same measures. These required pressure levels, especially for deep wells, have often exceeded the pressure at which many subterranean formations break down, thus giving rise to a problem of lost circulation. It also may happen that the required pressure level exceeds the capacity of the pumping equipment or the endurance of the well drilling and associated apparatus.
The present invention facilitates pumping of cement slurries at high flow rates and permits pumping of cement slurries in turbulent flow at significantly lower flow rates than would be possible using conventional cement slurry compositions, by adding to the said cement slurries a flow-property-improving and turbulence-inducing agent. The lower flow rates required for turbulence result in a corresponding reduction in the pump pressures required to force the cement slurry into place in the desired manner. Correspondingly, higher flow rates may be achieved with a reduced amount of required pressure and pumping capacity.
The present invention also permits increased use of fluid-loss control agents which have a viscosity-increasing effect on cement slurries with which they are employed. This thickening tendency otherwise requires use of only small amounts of fluid-loss control agents to avoid problems in pumping of an overly viscous cement slurry. Use of the flow-property-improving and turbulence-inducing additives of the present invention in conjunction with fluid-loss control agents produces a cement slurry having desirable flow properties, yet containing adequate quantities of the fluid-loss control agent.
2. Description of the Prior Art
U.S. Pat. No. 3,359,225 discloses an additive for Portland-type cements comprising polyvinylpyrrolidone and the sodium salt of naphthalene sulfonate condensed with formaldehyde. The additive reduces the friction encountered as the cement is flowed or pumped into place, and also permits the utilization of decreased quantities of water in the cement mixture. Harrison U.S. Pat. No. 3,409,080 discloses an aqueous hydraulic cement slurry containing an O,O-alkylene-O', O'-alkylene pyrophosphate-urea pyrolysis product and water-insoluble, water-dispersible polymeric material. The cement composition has improved properties whereby it may be injected in a state of turbulence without the expenditure of the amount of additional energy usually required to attain such a state. Kucera -- U.S. Pat. No. 3,465,824 discloses an aqueous hydraulic cement slurry containing a bisulfite-modified phenolformaldehyde condensation product which serves as a turbulence inducer to the slurry while being moved in a confined passageway. Hook et al. -- U.S. Pat. No. 3,465,825 discloses an aqueous cement slurry containing a turbulence-inducing agent comprising the lithium salt of the condensation product of mononaphthalene sulfonic acid and formaldehyde. Scott et. al. -- U.S. Pat. No. 3,511,314 discloses an aqueous hydraulic cement slurry containing a turbulence-inducing, fluid-loss control agent consisting of the reaction product of (1) an amino compound selected from the group consisting of polyalkylenepolyamines, polyalkylenimines, and their mixtures, and (2) an acidic compound selected from the group consisting of carboxylic acids, sulfonic acids, polymers having a carboxyl substituent, and polymers having a sulfonate substituent. Messenger -- U.S. Pat. No. 3,558,335 discloses cement compositions comprising hydraulic cement in admixture with a turbulence inducer and silica or diatomaceous earth particles. A number of known turbulence inducers are set out, any of which may be employed in the compositions disclosed.